<!DOCTYPE HTML>
<html>
<head>
<meta charset="utf-8">
<title>无标题文档</title>
<script>
//alert('abc'+'def'+12);
//alert('abc'+12+5+'def');
alert('abc'+(12+5)+'def');

//'abc'+12+5+'def'
//'abc12'+5+'def'
//'abc125'+'def'
//'abc125def'
</script>
</head>

<body>
</body>
</html>
